Acoustic ink printing is a promising direct marking technology because it does not require the nozzles or the small ejection orifices which have been a major cause of the reliability and pixel placement accuracy problems that conventional drop on demand and continuous stream ink jet printers have experienced.
It has been shown that acoustic ink printers having printheads comprising acoustically illuminated spherical focusing lenses can print precisely positioned picture elements ("pixels") at resolutions which are sufficient for high quality printing of relatively complex images. See, for example, the copending and commonly assigned U.S. patent applications of Elrod et al, which were filed Dec. 19, 1986 under Ser. No. 944,490, now U.S. Pat. No. 4,751,529, Ser. No. 944,698, now U.S. Pat. No. 4,751,530, and Ser. No. 944,701 on "Microlenses for Acoustic Printing", "Acoustic Lens Arrays for Ink Printing" and "Sparse Arrays for Acoustic Printing", respectively. It also has been found that such a printer can be controlled to print individual pixels of different sizes so as to impart, for example, a controlled shading to the printed image. See, another copending and commonly assigned U.S. patent application of Elrod et al, which was filed Dec. 19, 1986 under Ser. No. 944,286 on "Variable Spot Size Acoustic Printing".
Although acoustic lens-type droplet ejectors are favored for acoustic ink printing at the present time, there are other types of droplet ejectors which may be utilized, including (1) piezoelectric shell transducers, such as described in Lovelady et al U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on a "Liquid Drop Emitter," and (2) interdigitated transducers (IDT's), such as described in a copending and commonly assigned Quate et al U.S. patent application, which was filed Jan. 5, 1987 under Ser. No. 946,682 on "Nozzleless Liquid Droplet Ejectors" now U.S. Pat. No. 4,697,195 as a continuation of application Ser. No. 776,291 filed Sept. 16, 1985 (now abandoned). Additionally, it should be understood that acoustic ink printing technology is compatible with various printhead configurations, including (1) single ejector embodiments for raster scan printing, (2) matrix configured arrays for matrix printing, and (3) several different types of pagewidth arrays, ranging from (i) single row, sparse arrays for hybrid forms of parallel/serial printing, to (ii) multiple row staggered arrays with individual ejectors for each of the pixel positions or addresses within a pagewidth address field (i. e., single ejector/pixel/line) for ordinary line printing.
To carry out acoustic ink printing with any of the aforementioned droplet ejectors, each of the ejectors launches a converging acoustic beam into a pool of ink, such that the beam converges to focus at or near the free surface (i.e., the liquid/air interface) of the pool. The radiation pressure this beam exerts against the free surface of the ink is modulated, such that it makes brief controlled excursions to a sufficiently high pressure level to overcome the restraining force of surface tension. As a result, individual droplets of ink are ejected from the free ink surface on command, with sufficient velocity to deposit the droplets on a nearby recording medium.
As will be appreciated, polychromatic or "color" acoustic printing introduces a new set of challenges. It is performed by printing a plurality of monochromatic color separations of an image (cyan, magenta and yellow are the "primary colors" for subtractive color) in substantial registration with each other. Furthermore, it often is desirable to have the capacity to print a black separation, so the composition of a polychromatic image typically involves the printing of up to four different color separations in superimposed registration. These color separations can be printed by separate printheads, but a significant cost savings may be realized if provision is made for printing them with a single printhead. Additionally, a diluent may be used in some cases to provide an additional means for shading the images.